The present invention relates to the synthesis of unsaturated aldehydes from acetylenic alcohols and conjugated diolefins. More particularly, it relates to the synthesis of unsaturated aldehydes from the reaction of propargyl alcohol with conjugated diolefins.
Numerous 2+4 cycloadditions have been known since long before the advent of the pericyclic theory; they are among the most powerful of synthetic reactions. The most important of these is the Diels-Alder reaction. Although the Diels-Alder reaction occurs in the unsubstituted case, it is most successful when the diene and the alkene (referred to in this context as the dienophile) bear substituents of complementary electronic influence. Although these are most commonly an electron-donating group on the diene and an electron-withdrawing group on the dienophile, there are also a number of instances that illustrate inverse electron demand, that is electron-withdrawing groups on the diene and donating groups on the dienophile.
Unsaturated aldehydes can be prepared by the Diels-Alder reaction. An example of such a preparation is the reaction of acrolein with conjugated dienes. Acrolein is an .alpha., .beta.-unsaturated carbonyl compound which is a highly toxic lachrymatory liquid and which has a low boiling point (53.degree. C.). Examples of the preparation of unsaturated aldehydes by reaction of acrolein with conjugated dienes are the preparation of Cyclal C (reaction 1 below); the preparation of myraldene (reaction 2 below); and the preparation of cyclohexal (reaction 3 below). ##STR1##
Unfortunately, the highly toxic nature and low boiling point of acrolein make it a very dangerous compound. The fumes of acrolein are copiously produced at normal temperatures, i.e., above 0.degree. C. For this reason, restrictions have been placed on transporting acrolein, and acrolein is no longer generally available. However, the need to prepare unsaturated aldehydes still remains and is very important in many industries, such as in the fragrance industry.
Propargyl alcohol is a compound which is less toxic and has a higher boiling point (114-115.degree. C.) than acrolein. Propargyl alcohol can be converted to acrolein under particular reaction parameters. Catalysts used for such transformations include vanadium compounds, such as those which are discussed in Chabardes, et al., British Patent Specification 1,204,754, and a combination of titanium and copper catalysts, such as those described in Chabardes, et al., U.S. Pat. No. 4,749,814, the disclosures of each of which are incorporated herein by reference. There are, however, disadvantages to the reactions disclosed in these documents, which include, for example, that under the disclosed reaction parameters, acrolein polymerizes readily even in the presence of an inhibitor such as hydroquinone thereby limiting the useful yield of acrolein.